2cm=20mm, approx. 3/4-inch. I've experienced 2mm of adjustment turning a marginal ride into a sweet ride.

That said, if you're being professionally fit to a bike, you need to get the right size. Otherwise, it's your cash and your ride. Try the advice vs. your own thoughts, and go with what works better. If a 58cm bike feels good to you, get it. (I personally would look hardest at a 59, cuz that's what my known measurements tell me would fit. But Brand A's 58 or 60 could meet those numbers, too -- seen that happen!)